FIG. 7 shows a schematic construction of a conventional laser display. At first, rays of light emitted from laser light sources 101a-101c including three RGB colors (R: red, G: green, B: blue) are beam-expanded by an expander optical system 102 including collective lens 109a-109c. Next, expanded light rays are beam-formed by an integrator optical system including lenses and arrays of small lenses to provide a uniform illumination to a spatial light modulator 105. Herein, field lenses 104a-104c are adapted for converting the rays of light having been passed through the spatial light modulators 105a-105c into collected or collected light rays so as to make rays of light pass through an aperture of a projection lens 107 efficiently.
Then, an intensity modulation is executed to the beam-formed rays of light in the spatial light modulators 105a-105c in response to an input image signal, and the rays of light are combined in a dichroic prism 106. A combined ray of light is diverged in the projection lens 107 and projected to a screen 108. Accordingly, a two-dimensional image is displayed on the screen 108. In the display having this construction, the light sources for RGB respectively emit single color lights. Accordingly, an image having high color purity and brightness can be displayed by using laser light sources having appropriate wavelength.
However, such display has a problem of the so-called speckle noise which is likely to occur due to use of a laser light source having high coherency. The speckle noise is fine irregular noises which occur when rays of laser light are diffused on the screen 8 and the diffused light rays on respective portions of the screen 8 interfere with each other.
Conventionally, as disclosed in Patent Document 1, the speckle noise is eliminated by operating a motor 110 to oscillate a screen 108 or, as disclosed in Patent Document 2, by applying an external force to oscillate/rotate a diffuser. These methods change the speckle pattern within a time segment shorter than a display changing time which enables the human to see to average out a speckle pattern so that the observer cannot see the speckle noise. Further, Patent Document 3 discloses a method which prevents the speckle noise by changing a state of polarization of a laser light in a time frame and projects the laser light to a screen on which particles made of anisotropic crystals are applied.
Further, as described in Patent Document 4, the speckle noise can be reduced also by using a mobile diffuser, but there is a problem of noises such as motor driving sounds generated at the time when the diffuser is operated. To prevent sound noises caused by operation of a motor and the like, accordingly, Patent Document 5 discloses a method which prevents the speckle noise at a low cost without mechanical operation using a light diffusing optical system where microparticles are contained in a cell and the microparticles are electrically oscillated. However, in the case of using the speckle reducing method disclosed in Patent Document 5, the light rays diffused in the diffuser optical system are more likely to partially come outside of an image display area of the spatial light modulator as the distance between the spatial light modulator and the diffuser optical system becomes longer. As a result, the loss in the light intensity becomes larger, and the brightness of the screen lowers.
Furthermore, in the case of the diffuser optical system using microparticles, the microparticles in the cell are deviated so that a local diffusing angle and transmittance efficiency in the diffuser optical system differs at different locations. Therefore, as the distance between the diffuser optical system and the spatial light modulator comes closer, this deviated distribution of transmittance efficiency causes localization (luminosity irregularity) of light intensity on the spatial light modulator. The luminosity irregularity moves on the projection display following a movement of the diffuser optical system, and consequently runs over an image. Accordingly, in order to suppress image deterioration due to the luminosity irregularity, it is necessary to arrange the diffuser apart from the spatial light modulator with a predetermined distance.
In other words, speckle noise prevention optical systems, typically diffuser optical systems using microparticles, cannot eliminate the speckle noise and realize a high-resolution, bright image having no luminosity irregularity if the optical system is not set in an appropriate position with respect to the spatial light modulator. However, an appropriate positional relationship between the diffuser optical system and the spatial light modulator has not been discussed until now.
Patent Document 1:
Japanese Unexamined Patent Publication No. Sho55-65940
Patent Document 2:
Japanese Unexamined Patent Publication No. Hei6-208089
Patent Document 3:
Japanese Unexamined Patent Publication No. Hei3-109591
Patent Document 4:
Japanese Unexamined Patent Publication No. 2003-98476
Patent Document 5:
Japanese Unexamined Patent Publication No. Heill-218726